Advanced microbiology - not for everyone

The following excerpts were taken from the October Issue of the CDA JOURNAL, VOL 4 5 , Nº 10. Access the issue for more information about the importance of balancing oral ecology for human health.

The Mouth as a Microbial Habitat

The mouth is similar to other habitats in the body in supporting the growth of a natural and characteristic community of microorganisms (the oral microbiome) that also delivers benefits to the host (FIGURE 1)(8). The mouth is warm and moist and is supplied with a broad array of proteins and glycoproteins from saliva and gingival crevicular fluid (GCF) that are critical for key physiological functions (lubrication, host defenses, etc.), but which can also be exploited by microorganisms as nutrients. Therefore, a diverse range of viruses, bacteria, Archaea, fungi and even protozoa can be commonly isolated from the mouth, of which bacteria are the most numerous and diverse group — approximately 700 species have been identified. Of these species, only about half have been given official names, while 30 percent have yet to be cultivated in the laboratory(9,10). Any one person has approximately 200 microbial species naturally inhabiting his or her mouth.

These microorganisms colonize mucosal and dental surfaces and persist in the mouth by forming three-dimensional, structurally organized multispecies interactive communities termed biofilms(11–13). In general, desquamation ensures that the microbial load on mucosal surfaces is kept relatively low, but the mouth is a unique site in the body in that it provides nonshedding surfaces (teeth, dentures, implants) for microbial colonization. This can result in the accumulation of large numbers of microorganisms, particularly at stagnant and hard-to clean sites, unless patients practice effective oral hygiene. The biofilms that form on teeth have previously been referred to as dental plaque. A number of environmental factors influence the distribution and metabolic activity of the resident oral
microbiome (FIGURE 2).

The relationship between the oral microbiome and the host is not passive but is active and dynamic and as a result is susceptible to change (FIGURES 3A and 3B). There is active communication (“cross talk”) between the resident oral microbiota and host cells to avoid a damaging or excessive inflammatory response to these beneficial organisms. Some resident bacteria, especially streptococci, are involved in this cross talk and downregulate potentially proinflammatory host responses to members of the normal oral microbiota, such as the Gram-negative commensals, which could be damaging to host tissues(25). Despite this, the host is still able to retain the ability to respond to genuine microbial threats.

Resident oral bacteria make a major contribution to the general health of their host by regulating gastrointestinal and cardiovascular systems via the metabolism of dietary nitrate(26).

Approximately 25 percent of ingested nitrate is secreted in saliva, from where it is reduced to nitrite by commensal oral bacteria. Nitrite regulates blood flow, blood pressure and gastric integrity and is converted to nitric oxide in the acidified stomach. This has antimicrobial properties that contribute to the defense against enteropathogens and in the regulation of gastric mucosal blood flow and mucus formation.

These properties emphasize that it is essential to maintain a natural oral microbiome so as not to lose the beneficial functions of these resident oral microorganisms, and this has implications for treatment strategies. As already stated, the symbiotic relationship between the oral microbiome and the host is dynamic (FIGURE 3A) and will be affected by changes in lifestyle (e.g., diet, smoking, antibiotic treatment, etc.), decreased rates of saliva flow, inadequate oral hygiene and compromised host defenses that can alter microbe — microbe and ultimately microbe — host interactions (FIGURE 3B). Oral health is more than the absence of disease and needs active promotion and management(27).

Ecological Plaque Hypotheses: Implications for Treatment

The ecological plaque hypothesis in its original(54) and extended form(30), and the more recent polymicrobial synergy and dysbiosis model for periodontal disease(73), recognizes the direct and dynamic link between local environmental conditions in the mouth and the activity and composition of the biofilm community, so that any change to the host environment will induce a response in the microbiota and vice versa. A key principle of the original ecological plaque hypothesis, however, is that long-term prevention of dental diseases will only be achieved by interfering with the underlying changes in host environment that drive the deleterious shifts in the microbiota(54). This could be by improving oral hygiene practices to more effectively disrupt or remove biofilm by lifestyle changes, such as altering the diet or by using oral care products or other approaches that interfere with the drivers of dysbiosis and/or promote symbiosis (TABLE)(76,77). The repeated production of acid from the microbial fermentation of dietary sugars and the regular lowering of the pH in dental biofilms over time selects for acidogenic and acid-tolerating bacteria while inhibiting the growth of beneficial species
(FIGURE 5A). Approaches that could reduce microbial acid production include the use of oral care products containing inhibitors (TABLE).